Proper headspace in a firearm is of critical importance. This dimension relates to the distance between the aft end of the chamber of a firearm or cartridge contained within the chamber of that firearm, and the forward end of the bolt or other mechanism which serves to retain the cartridge within the firearm chamber. Headspace is properly measured when the bolt or mechanism is at its forwardmost travel and the chamber is closed.
When a firearm is fired, the cartridge will tend to expand to the limits of the available volume within the firearm chamber. If this volume is too great, as may be caused by too much headspace, the cartridge may burst, causing damage to the firearm or possible injury to the shooter. If headspace is too short, it may prove impossible to completely insert a cartridge into the chamber of the firearm and completely close and lock the bolt.
Obviously, the dimensions of the cartridge to be used in the firearm have a great bearing on the headspace, and properly manufactured new cartridges will precisely fit the firearm for which they were manufactured. However, many persons engaged in the firearms or shooting hobby reload their own previously fired cartridges for purposes of economy, special loads or bullets, etc. Such previously fired cartridges may be beyond proper tolerances for proper headspace, for a variety of reasons.
A cartridge may provide too little headspace if it has been expanded due to previous use. Such a cartridge may be resized, but if the resizing operation is not carried out precisely, it may lead to too much headspace and the potential problems associated with that situation. As can be seen, the exact determination of cartridge headspace is vital, but is difficult to measure directly. The need arises for a device which may easily and precisely determine the exact headspace of a given size of firearm cartridge.